Field of the Invention
The present invention relates to an image forming apparatus with an electromagnetic induction heating type fixing unit to fix a formed image.
Description of the Related Art
An electrophotographic image forming apparatus generally includes a fixing unit which adds heat and pressure to fix a toner image transferred on a printing medium to a sheet such as paper. Recently, a method of heating by electromagnetic induction has started to be used in fixing units.
An image forming apparatus has the problem that the temperature at the end of a fixing roller rises more than necessary when an image is formed on a printing medium of a relatively small size such as B5 size because no sheet removes heat from the fixing roller in a non-passage area where the sheet does not pass at the end of the fixing roller of the fixing unit. To solve this problem, Japanese Patent Laid-Open No. 2004-325678 discloses an arrangement which uses a Curie material for the fixing roller to suppress a temperature rise in the non-passage area. The Curie material is a magnetic shunt alloy having a characteristic in which magnetism abruptly drops when the temperature reaches the Curie temperature. In an area where magnetism drops, induction heating hardly occurs, decreasing the amount of generated heat.
If an abnormal status, such as a short circuit, occurs in a coil for heating by electromagnetic induction, a large current may flow through the coil via a switching element to damage the power source device. To prevent this, Japanese Patent Laid-Open No. 2000-223253 discloses an arrangement which detects an output current flowing through the coil, and when the output current enters an overcurrent state, the image forming apparatus determines that the fixing unit has become abnormal, and stops the operation.
The impedance of a fixing unit using a Curie material has a characteristic in which it abruptly varies near the Curie temperature. Note that an impedance in an area where the temperature of the fixing unit is higher than the Curie temperature is lower than an impedance in an area where it is lower than the Curie temperature. If output power to the fixing unit is constant, a current flowing through the fixing unit increases abruptly when the temperature of the fixing unit exceeds the Curie temperature.
If a threshold to detect the abnormal status of an output current to the fixing unit is determined by an output current at the Curie temperature or higher, output power to the fixing unit becomes excessively large in an abnormal status at the Curie temperature or lower. An overpower detection circuit may be attached to the power source device to prevent excessive output power. However, a complicated hardware circuit is required, increasing the circuit area and substrate cost.